Accretion definition, an increase by natural growth or by gradual external addition; growth in size or extent. See more.
Alveoli are tiny sacs within our lungs that allow oxygen and carbon dioxide to move between the lungs and bloodstream. Learn more about how they function and quiz your knowledge at the end.
The alveolar sacs consist of many alveoli and are composed of a single layer of epithelial tissue. There are about 700 million alveoli in the adult lung. The average, healthy adult lungs hold about 13 to 13.5 pints of air (about six liters). The alveoli are considered the functional unit of the lung. In the spaces between the alveoli of the lungs is elastic connective tissue which is important for exhalation. The alveoli are surrounded by a network of pulmonary capillaries.